Rotavirus infection is the leading cause of severe acute diarrhea among young children worldwide. An estimated 527,000 children aged <5 years die from rotavirus diarrhea each year, with >85% of these deaths occurring in low-income countries of Africa and Asia. Two licensed rotavirus vaccines have shown efficacy of 85%-98% against severe rotavirus diarrhea in trials conducted in the Americas and Europe, and they have been introduced into routine immunization programs in 11 countries in these regions and in Australia. Additional trials of these vaccines are ongoing to assess efficacy in low-income countries of Asia and Africa, where vaccine performance might be affected by factors such as concurrent enteric infections, greater prevalence of malnutrition, and a greater prevalence of unusual rotavirus strains. Results of these additional trials are expected within the next 1-2 years. To collect epidemiologic and burden-of-disease data that could form the basis of vaccination policy worldwide, beginning in 2001, the World Health Organization (WHO), in collaboration with partners, established networks of hospital-based sentinel surveillance sites for detection of rotavirus diarrhea and characterization of rotavirus strains. This report presents an analysis of results from the WHO surveillance networks for 2001-008, which indicated that approximately 40% of diarrhea hospitalizations among children aged <5 years worldwide were attributed to rotavirus infection. The most common rotavirus strains found were G1, G2, G3, G4, and G9, and the distribution of strains varied markedly across regions. These data demonstrate the substantial burden of rotavirus diarrhea worldwide and highlight the potential health impact of vaccination.